Using Calculators in Large Format Photography - Part I

By Frank Sirona

Part I: Depth of Field

Large format photography requires a different approach to technical challenges compared to modern digital cameras. While digital photographers rely on autofocus systems and live view to check depth of field, large format photographers must calculate and visualize these elements before exposing the film. This is where field calculators become invaluable tools.

Understanding Depth of Field in Large Format

Depth of field (DOF) is the zone of acceptable sharpness in a photograph, extending from the nearest to the farthest objects that appear in focus. In large format photography, controlling depth of field is both more critical and more challenging than in smaller formats, due to the limited depth of field at typical working apertures and the ability to use camera movements.

Several factors affect depth of field:

• Aperture - Smaller apertures (higher f-numbers) increase depth of field
• Focal length - Shorter lenses provide greater depth of field at the same aperture
• Subject distance - Greater distances increase depth of field
• Circle of confusion - The acceptable blur size, which varies by format and viewing conditions

The Challenge of Calculating DOF

Calculating depth of field in the field can be complex. The mathematical formulas involve multiple variables and require precision. While it's possible to memorize or estimate, using a calculator ensures accuracy, especially when precise focus placement is critical.

The traditional depth of field formula is:

DOF = (2 × N × c × s²) / (f² - N × c × s)

Where:

• N = f-number (aperture)
• c = circle of confusion
• s = subject distance
• f = focal length

This formula quickly becomes unwieldy in the field, which is where calculators prove their worth.

Types of Field Calculators

Rotating Disc Calculators

These classic calculators feature rotating discs with scales for focal length, aperture, and distance. By aligning the appropriate values, you can read the near and far limits of acceptable focus directly from the scale. They're compact, require no batteries, and are quick to use once you're familiar with them.

Slide Rule Style Calculators

Similar in principle to rotating discs, slide rule calculators use linear scales instead of circular ones. Some photographers prefer these for their larger, easier-to-read scales.

Digital Calculator Apps

Modern smartphone apps can calculate depth of field precisely and quickly. They're particularly useful for complex scenarios or when you need to explore multiple what-if scenarios. However, they require charged batteries and can be difficult to read in bright sunlight.

The Scheimpflug Principle

One unique aspect of large format photography is the ability to use camera movements—particularly tilt and swing—to control the plane of focus. The Scheimpflug principle states that the lens plane, the film plane, and the plane of sharp focus all intersect at a common line.

By tilting or swinging the lens or back standard, you can align the plane of focus with your subject, potentially achieving front-to-back sharpness at larger apertures than would otherwise be required. This is particularly useful for landscape photography where you want everything from near foreground to distant background in focus.

While calculators can't directly compute Scheimpflug adjustments, understanding depth of field calculations helps you determine when camera movements might be beneficial and how much DOF you'll gain from their use.

Practical Application

Here's a typical workflow using a field calculator:

1. Identify your near and far focus points - Determine what elements in your scene must be sharp
2. Measure or estimate distances - Use a tape measure or estimate the distances to these points
3. Input your lens focal length - Set the calculator to your lens
4. Calculate required aperture - Find what f-stop provides the needed DOF
5. Determine focus point - Calculate the hyperfocal distance or optimal focus point
6. Set up your shot - Apply the calculated values to your camera

The Hyperfocal Distance

A related concept essential to landscape photography is the hyperfocal distance—the focus distance that maximizes depth of field for a given aperture and focal length. When focused at the hyperfocal distance, everything from half that distance to infinity appears acceptably sharp.

Calculators make finding the hyperfocal distance straightforward, and many include dedicated scales for this purpose.

Limitations and Considerations

While calculators are valuable tools, they have limitations:

• They assume ideal conditions and may not account for lens aberrations
• Circle of confusion standards are subjective and vary by viewing conditions
• Camera movements affect the calculations in ways the calculator can't account for
• Close-up work requires special considerations for effective aperture

Despite these limitations, field calculators remain indispensable tools for large format photographers who want to work methodically and achieve predictable, sharp results.

Conclusion

Mastering depth of field calculations with field calculators is a fundamental skill for large format photographers. These simple tools enable you to work with confidence, ensuring that your carefully composed images have the sharpness exactly where you need it. In Part II, we'll explore another essential calculator application: exposure compensation for close-up work.